Carbohydrated-Mediated Sperm-Egg Interactions In Mammals

Abstract

The fertilization pathway in mice consists of several steps that must occur in a compulsory order prior to fusion of sperm and egg (reviewed in Wassarman, 1987a,b, 1988a). The steps include: (i) loose “attachment” and then tight species-specific “binding” of acrosomeintact sperm to the unfertilized egg’s extracellular coat, or zona pellucida; (ii) completion of the acrosome reaction, an exocytotic event involving fusion of outer acrosomal and plasma membranes, by bound sperm; (iii) penetration of the zona pellucida by bound acrosomereacted sperm (perhaps, by using a sperm proteinase, “acrosin”); (iv) fusion of acrosomereacted sperm with the egg’s plasma membrane. Within a few minutes of sperm-egg fusion, the zona pellucida is altered (“zona reaction”) such that it becomes refractory to both binding of sperm and penetration by sperm; this constitutes a secondary block to polyspermy. These changes in the nature of the zona pellucida are thought to be brought about by cortical granule enzymes released into the extracellular coat as a consequence of the cortical reaction (fusion of cortical granule and plasma membranes). Thus, the steps in the fertilization pathway include species-specific cellular recognition, intracellular and intercellular membrane fusions, and enzyme-catalyzed modifications of cellular investments.